When processing a material, a measurement of a surface cleanliness (e.g., quantity or quality of contaminant) of the material may be required. For example, verification of an adequate surface cleanliness is useful when bonding surfaces of composite objects to ensure sufficient adhesion or bond strength between composite bonding surfaces. Even low levels of contaminant (e.g., 1 mg/ft2) can greatly reduce the bond strength and cause structural failure.
Sources of contamination on the bonding surface of a composite object include, but are not limited to, contamination after a peel ply material is removed from the bonding surface but prior to application of the adhesive, and application of peel ply material other than what was specified (i.e., application of the wrong type of peel ply material).
Typically, a peel ply material made as a polyester cloth or fabric, is baked onto a bonding surface when the material to be bonded is cured in an autoclave. Prior to bonding, the peel ply material is removed. However, contamination of the bonding surface may occur after the peel ply material is removed, from contaminants such as silicones, teflons, hydraulic oils, lubricants, engine oils, and the like.
Furthermore, the wrong type of peel ply material than specified for the bonding surface may be inadvertently used, causing contamination and a weak bond. For example, residues of silicone-impregnated polyester fabrics (such as the commercially available silicon release blue) and nylon-based fabrics are known to cause weak bonding.
Infrared (IR) spectroscopy has been previously used for surface chemistry measurements and cleanliness verification, as disclosed for example in U.S. patent application Ser. No. 10/329,734 to Shelley et al., published Dec. 18, 2003. However, some surfaces, such as the peel ply bonding surface on graphite epoxy composite materials, are difficult to measure by this method because of roughness and/or absorption characteristics of the surface. Graphite fibers, for example, absorb the light from most spectroscopic methods, such as those using IR light, and are therefore not amenable to IR spectroscopy analysis.
Another previous method of detecting contaminants on composite surfaces has been diamond attenuated total reflectance IR spectroscopy in which a diamond crystal is forced against the surface to be tested (e.g., with about 2,000 pounds of force), and any residue on the surface is transferred to the diamond crystal for measurement. However, this method is highly inefficient as it is slow, tests only a very small area at a time (e.g., 2 mm spot/test), and measurements cannot be made in real-time.
Because the level of a contaminant that is acceptable can differ based upon the particular contaminant and surface to be processed, it is desirable to be able to identify specific individual contaminants on a surface to be processed.
It is known that a bonding surface may be treated with plasma such that the plasma electrons and ions clean and derivatize the surface prior to adding adhesive to the surface. If the surface is not sufficiently treated to the required levels of cleanliness and/or derivation, the composite objects may not properly adhere. Reworking surfaces that have failed to adhere increases the time spent on processing and cost. The rework process may also generate additional waste disposal in many cases.
As a result, there is a need for a sensitive, real-time measurement of surface cleanliness and surface chemistry of composite bonding surfaces to verify effectiveness of the plasma treatment prior to performing the bonding operation.